I am trying to get better at using documentation to learn about Linux programs, but that "Synopsis" section is intimidating to new users. It literally looks like gibberish compared to man locate or man free.

So far, I understand that square brackets mean optional and nested brackets mean options in optional. But how am I supposed to induce a valid command with that?

I am not asking for help with xargs here. I am looking for help interpreting a man page to understand complicated commands. I want to stop making Google-indexed web blogs and personal help from others my first approach to learning Linux commands.

11 Answers
11

The manpager

When you open a manpage using the man command, the output will be displayed/rendered by the less or more commands, or any other command that will be set as your pager(manpager).

If you are using Linux you are probably served with your man infrastructure already configured to use /usr/bin/less -is (unless you installed some minimal distro) as man(1), explain on it's Options section:

On FreeBSD and OpenBSD is just a matter of editing the MANPAGER environment variable since they will mostly use more, and some features like search and text highlight could be missing.

There is a good answer to the question of what differences more, less and most have here(never used most). The ability to scroll backwards and scroll forward by page with Space or both ways by line with ↓ or ↑(also, using vi bindings j and k) is essential while browsing manpages. Press h while using less to see the summary of commands available.

And that's why I suggest you to use less as your man pager. less have some essential features that will be used during this answer.

How is a command formatted?

Utility Conventions: The Open Group Base Specifications Issue 7 - IEEE Std 1003.1, 2013 Edition. You should visit that link before trying to understand a manpage. This online reference describes the argument syntax of the standard utilities and introduces terminology used throughout POSIX.1-2017 for describing the arguments processed by the utilities. This will also indirectly get you updated about the real meaning of words like parameters, arguments, argument option...

The head of any manpage will look less cryptic to you after understanding the notation of the utility conventions:

Apropos works with regular expressions by default, (man apropos, read the description and find out what -r does), and on this example I'm looking for every manpage where the description starts with "report".

To look for information related with reading standard input/output processing and reaching xargs as a possible option:

Always read the DESCRIPTION before starting

Take a time and read the description. By just reading the description of the xargs command we will learn that:

xargs reads from STDIN and executes the command needed. This also means that you will need to have some knowledge of how standard input works, and how to manipulate it through pipes to chain commands

The default behavior is to act like /bin/echo. This gives you a little tip that if you need to chain more than one xargs, you don't need to use echo to print.

We have also learned that unix filenames can contain blank and newlines, that this could be a problem and the argument -0 is a way to prevent things explode by using null character separators. The description warns you that the command being used as input needs to support this feature too, and that GNU find support it. Great. We use a lot of find with xargs.

xargs will stop if exit status 255 is reached.

Some descriptions are very short and that is generally because the software works on a very simple way. Don't even think of skipping this part of the manpage ;)

Other things to pay attention...

You know that you can search for files using find. There is a ton of options and if you only look at the SYNOPSIS, you will get overwhelmed by those. It's just the tip of the iceberg. Excluding NAME, SYNOPSIS, and DESCRIPTION, you will have the following sections:

AUTHORS: the people who created or assisted in the creation of the
command.

BUGS: lists any known defects. Could be only implementation limitations.

ENVIRONMENT: Aspects of your shell that could be affected by the command, or variables that will be used.

EXAMPLES or NOTES: Self explanatory.

REPORTING BUGS: Who you will have to contact if you find bugs on this tool or in its documentation.

COPYRIGHT: Person who created and disclaimers about the software. All related with the license of the software itself.

SEE ALSO: Other commands, tools or working aspects that are related to this command, and could not fit on any of the other sections.

You will most probably find interesting info about the aspects you want of a tool on the examples/notes section.

Example

On the following steps I'll take find as an example, since it's concepts are "more simple" than xargs to explain(one command find files and the other deals with stdin and pipelined execution of other command output). Let's just pretend that we know nothing(or very little) about this command.

I have a specific problem that is: I have to look for every file with the .jpg extension, and with 500KiB (KiB = 1024 byte, commonly called kibibyte), or more in size inside a ftp server folder.

First, open the manual: man find. The SYNOPSIS is slim. Let's search for things inside the manual: Type / plus the word you want (size). It will index a lot of entries -size that will count specific sizes. Got stuck. Don't know how to search with "more than" or "less than" a given size, and the man does not show that to me.

Let's give it a try, and search for the next entry found by hitting n. OK. Found something interesting: find\( -size +100M -fprintf /root/big.txt %-10s %p\n \). Maybe this example is showing us that with -size +100M it will find files with 100MB or more. How could I confirm? Going to the head of the manpage and searching for other words.

Again, let's try the word greater. Pressing g will lead us to the head of the manpage. /greater, and the first entry is:

Numeric arguments can be specified as
+n for **greater** than n,
-n for less than n,
n for exactly n.

Sounds great. It seems that this block of the manual confirmed what we suspected. However, this will not only apply to file sizes. It will apply to any n that can be found on this manpage (as the phrase said: "Numeric arguments can be specified as").

Good. Let us find a way to filter by name: g/insensitive. Why? Insensitive? Wtf? We have a hypothetical ftp server, where "that other OS" people could give a file name with extensions as .jpg, .JPG , .JpG. This will lead us to:

-ilname pattern
Like -lname, but the match is case insensitive. If the -L
option or the -follow option is in effect, this test returns
false unless the symbolic link is broken.

However, after you search for lname you will see that this will only search for symbolic links. We want real files. The next entry:

-iname pattern
Like -name, but the match is case insensitive. For example, the
patterns `fo*' and `F??' match the file names `Foo', `FOO',
`foo', `fOo', etc. In these patterns, unlike filename expan‐
sion by the shell, an initial '.' can be matched by `*'. That
is, find -name *bar will match the file `.foobar'. Please note
that you should quote patterns as a matter of course, otherwise
the shell will expand any wildcard characters in them.

Great. I don't even need to read about -name to see that -iname is the case insensitive version of this argument. Lets assemble the command:

Command:find /ftp/dir/ -size +500k -iname "*.jpg"

What is implicit here: The knowledge that the wildcard ? represents "any character at a single position" and * represents "zero or more of any character". The -name parameter will give you a summary of this knowledge.

Tips that apply to all commands

Some options, mnemonics and "syntax style" travel through all commands making you buy some time by not having to open the manpage at all. Those are learned by practice and the most common are:

Generally, -v means verbose. -vvv is a variation "very very verbose" on some software.

Following the POSIX standard, generally one dash arguments can be stacked. Example: tar -xzvf, cp -Rv.

Generally -R and/or -r means recursive.

Almost all commands have a brief help with the --help option.

--version shows the version of a software.

-p, on copy or move utilities means "preserve permissions".

-y means YES, or "proceed without confirmation" in most cases.

Note that the above are not always true though. For example, the -r switch can mean very different things for different software. It is always a good idea to check and make sure when a command could be dangerous, but these are common defaults.

Default values of commands.

At the pager chunk of this answer, we saw that less -is is the pager of man. The default behavior of commands are not always shown at a separated section on manpages, or at the section that is most top placed.

You will have to read the options to find out defaults, or if you are lucky, typing /pager will lead you to that info. This also requires you to know the concept of the pager(software that scrolls the manpage), and this is a thing you will only acquire after reading lots of manpages.

Why is that important? This will open up your perception if you find differences on scroll and color behavior while reading man(1) on Linux(less -is pager) or FreeBSD man(1) for example.

And what about the SYNOPSIS syntax?

After getting all the information needed to execute the command, you can combine options, option-arguments and operands inline to make your job done. Overview of concepts:

Options are the switches that dictates a command behavior. "Do this"
"don't do this" or "act this way". Often called switches.

Option-arguments are used on most cases when an option isn´t
binary(on/off) like -t on mount, that specifies the type of a
filesystem(-t iso9660, -t ext2). "Do this with closed eyes" or
"feed the animals, but only the lions". Also called arguments.

Operands are things you want that command to act upon. If you use
cat file.txt, the operand is a file inside your current
directory, and it´s contents will be shown on STDOUT. ls is a
command where an operand is optional. The three dots after the operand
implicitly tells you that cat can act on multiple operands(files) at
the same time. You may notice that some commands have set what type of
operand it will use. Example: cat [OPTION] [FILE]...

When will this method not work?

Manpages that have no examples

Manpages where options have a short explanation

When you use generic keywords like and, to, for inside the manpages

Manpages that are not installed. It seems to be obvious but, if you don't have lftp (and its manpages) installed you can't know that is a suitable option as a more sophisticated ftp client by running man -k ftp

In some cases the examples will be pretty simple, and you will have to make some executions of your command to test, or in a worst case scenario, Google it.

Other: Programming languages and it's modules:

If you are programming or just creating scripts, keep in mind that some languages have it's own manpages systems, like perl(perldocs), python(pydocs), etc, holding specific information about methods/funcions, variables, behavior, and other important information about the module you are trying to use and learn. This was useful to me when i was creating a script to download unread IMAP emails using the perl Mail::IMAPClient module.

You will have to figure out those specific manpages by using man -k or searching online. Examples:

[root@host ~]# pydoc sys
Help on built-in module sys:
NAME
sys
FILE
(built-in)
MODULE DOCS
http://www.python.org/doc/current/lib/module-sys.html
DESCRIPTION
This module provides access to some objects used or maintained by the
interpreter and to functions that interact strongly with the interpreter.
...again, another full-featured manpage with interesting info...

Or, the help() funcion inside python shell if you want to read more details of some object:

nwildner@host:~$ python3.6
Python 3.6.7 (default, Oct 21 2018, 08:08:16)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> help(round)
Help on built-in function round in module builtins:
round(...)
round(number[, ndigits]) -> number
Round a number to a given precision in decimal digits (default 0 digits).
This returns an int when called with one argument, otherwise the
same type as the number. ndigits may be negative.

The following conventions apply to the SYNOPSIS section and can be used
as a guide in other sections.
bold text type exactly as shown.
italic text replace with appropriate argument.
[-abc] any or all arguments within [ ] are optional.
-a|-b options delimited by | cannot be used together.
argument ... argument is repeatable.
[expression] ... entire expression within [ ] is repeatable.

As for how you're supposed to write a valid command from that, well, you're not. The synopsis is useful once you know how a command works. It can help you refresh your memory. In order to understand how the command works, you should read the man page. Especially the descriptions of the options and the examples section.

Sometimes the synopsis is enough. For example, in man ls:

SYNOPSIS
ls [OPTION]... [FILE]...

Other times, it is useless unless you already know how to use the command in question. For example, man dd:

dd [OPERAND]...
dd OPTION

So, in conclusion, don't worry if you don't get the synopsis. That's normal. Read the man page itself.

man pages normally are displayed with less nowadays. That makes it possible to search through them. I would not bother with the synopsis, especially not because you have a particular commandline that you want to understand.

Hit the / and start typing -I and then Enter. The first hit will be in the synopsis, the second (use n for next) gets you the detailed explanation for -I.

You need information on not only xargs but also bash and [ (this may be in the test manpage). You may also need information on your shell (probably also bash) for the quoting rules, since your command includes a complex quoted string. I can already tell you that the quoted argument is wrong (and wrong in a way that will only show up when you encounter a file with spaces in the name); the inner "{}" should probably be '{}'.

So first you would refer to the xargs manpage and see [-I replace-str] for what -I {} means, and [command [initial-arguments]] for what bash and everything after it means. Then you would refer to the bash manpage for what -c does, etc.

1) If you're interested in a gnu utility, especially ones like sed and grep, sometimes, using the info command will bring up a greatly expanded version of the command information. sed, for instance, has a detailed section on how to write regular expressions and another section with some very complex usage examples.

2) It's a "manual". A manual is primarily designed to help you remember the details of something you already understand. It's designed so you can get the details you need quickly and get out. (And way too many have no usage examples or only trivial ones.)

When I need to learn something new, even a small feature of a command that isn't clear to me, I go to the web and search using the best keywords I can think of (e.g. Linuxxargs) and add the word howto, examples, or tutorial. This is very often quite productive.

I usually use duckduckgo because it maintains my privacy, but if I need more control of my searches, I use Google because I can tell it just to search within one website or to only return results from the last year. (It has many other Advanced Search options. You can Google them. ;) )

Another tip:

For commands I reference often, I save the man page in a text file

man bash > bashman.txt

and load that file into another window in my text editor so I can flip back and forth, copy and paste, etc.. I usually make the file read only, so I don't end up mangling it by accident, but since it's my personal copy, I could edit it in any way I want to, adding tags so I can find a section more easily or even adding my own notes or examples.

The only drawback to this is that the original man page may be updated and my copy is static.

To get a quick help with your specific command, you can use Explain Shell. E.g. your command. After getting the first high-level understanding how this works, you should proceed with manpages as other answers recommend.

cheat will display the frequently used option for a command line. Also you can add your own command + option to be displayed through cheat for a specific command.

bropages tool provides some example for a command line , it is possible to add you own example or upvote / downvote the given example after getting the verification code through bro thanks. The must voted command will appear on the top of the bro page.

I suggest tldr for a comprehensive man-alike tool. Simplified and community-driven man pages. In ubuntu you can install it via snap, but it has versions for other distros as well. It also provides you with several common example uses. 100% recomended.

> I am looking for help interpreting a man page to understand complicated commands.

I think that's the misunderstanding here. What makes Linux/UNIX so powerful is that you can construct very long and effective commands by e.g. redirecting the stdout of one command (here, locate) to the stdin of another (here, xargs) through a pipe (|). Hence, as Random832 correctly said, you won't find a single manpage that explains what your example command does.

I recommend that you read a Linux shell scripting guide; the Advanced Bash-Scripting Guide is an excellent document. Please don't be intimidated by it; mastering Linux requires years and is a never-ending process, but you can learn the basics in a reasonable amount of time.

Then, refer to manpages when you need to know the details of a specific command.

Thank you for your interest in this question.
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